A. Field of the Invention
The present invention relates generally to fuel management systems for internal combustion engines and, more particularly, to an air/fuel ratio management system for an internal combustion engine utilizing a gaseous fuel.
B. Description of the Prior Art
In order to obtain optimum engine operation, modern day internal combustion engines monitor the air/fuel ratio of the fuel charge to the engine to optimize engine performance. Careful monitoring of the air/fuel ratio of the fuel charge is necessary to obtain best fuel economy and low engine emissions. As used herein, "fuel charge" is the mixture of fuel and air provided to the engine for combustion.
There is also a trend to employ gaseous fuel with internal combustion engines. As used herein, a "gaseous fuel" means a fuel which is in the gaseous state at standard temperature and pressure. A few examples of a gaseous fuels used with internal combustion engines are: compressed natural gas, liquid natural gas, and liquid petroleum gas. Gaseous fuel internal combustion engines which carefully manage the air/fuel ratio are of particular benefit in heavy duty trucks.
In order to monitor the air/fuel ratio for a gaseous fuel internal combustion engine, typically an air/fuel ratio sensor, commonly an oxygen sensor, is exposed to combustion products in the exhaust gas stream of the engine. The amount of oxygen in the combustion products of a fuel charge is indicative of the air/fuel ratio of that fuel charge just prior to combustion.
One disadvantage of an air/fuel ratio sensor in the exhaust stream is that feedback to a control system is limited to the fuel charge which has already been expended. Consequently, the delay imposed between engine combustion and air/fuel ratio sensor detection results in less than optimum engine performance--especially during transient engine operating conditions.
Some existing gaseous engine systems add more sensors to provide improved control. Specifically, a gas mass flow rate sensor is added to the gaseous fuel line and an air mass flow rate sensor is added to the air intake pathway upstream of the engine manifold. Typically, an input air/fuel ratio is determined from these extra sensors which can be used to control air/fuel ratio alone, or in conjunction with the more traditional exhaust air/fuel ratio sensor. Although these systems usually provide more effective control over the engine's air/fuel ratio, it is only at the expense of these additional sensors. Furthermore, these sensors are detrimental to the reliability of this system because they increase the number of likely failure points.
Existing systems also suffer from other limitations. For example, the poppet-type valve commonly used to regulate fuel flow in gaseous engines requires a relatively high gaseous fuel pressure to operate effectively. However, lower fuel line pressure is more cost effective for some applications.
Consequently, a need remains for a gaseous internal combustion engine that employs the more effective air/fuel ratio control obtained with air/fuel ratio sensing at the engine input, but without adding two extra sensors. Also, if it is desirable to use a low pressure gaseous fuel source with such an engine, then the problem of effectively regulating low pressure gaseous fuel flow still needs to be solved.